In the past, in a light-emitting element device using a short-wavelength light-emitting element such as an ultraviolet light-emitting element or blue light-emitting element, this short-wavelength light-emitting element 1 such as an ultraviolet light-emitting element or blue light-emitting element has been covered, for example, with epoxy resin 2 as shown in FIG. 5.
Also, in a light-emitting element device which uses this short-wavelength light-emitting element 1 such as an ultraviolet light-emitting element or blue light-emitting element and is intended to obtain visible light through wavelength conversion, an element such as phosphorus has been mixed into this covering epoxy resin 2 so as to obtain yellow light.
Further, calcium fluoride crystals (fluorite) are generally known to be highly resistant to ultraviolet rays such as KrF and ArF excimer lasers (Published Japanese Patent Application No. 2001-33379).
However, this short-wavelength light-emitting element 1 such as an ultraviolet light-emitting element or blue light-emitting element is high in the energy of light, which thus decomposes couplings of the epoxy resin 2 or the like covering this short-wavelength light-emitting element 1 and causes devitrification. As a result, regarding a light-emitting element device using this short-wavelength light-emitting element 1, there has been such inconvenience in which light-emitting efficiency lowers noticeably or similar effects are caused, as it is used.
Further, when in order to perform wavelength conversion in a light-emitting element device using this short-wavelength light-emitting element 1 an element such as phosphorus, for example, is mixed into the covering epoxy resin 2 to obtain a light-emitting element device emitting yellow light, with the energy of this short-wavelength light, the decomposition of the epoxy resin 2 progresses as it is used, light-emitting efficiency deteriorates, and this phosphorus atom becomes directly attached to the surface of wiring of a diode constituting this short-wavelength light-emitting element 1, corroding this wiring and causing an inconvenience in which reliability is lowered greatly.
Meanwhile, the development of calcium fluoride which is great in hardness, excellent in acid resistance, small in dislocation density, and small in strain amount is being hoped for.